How to add Meta-Data on Plugin Activation

register_activation_hook(__FILE__, 'mpm_activation');
 
function mpm_activation(){
  global $post;
   $querystr = "
    SELECT wposts.* 
    FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta
    WHERE wposts.ID = wpostmeta.post_id 
    AND wpostmeta.meta_key = 'price'
    OR  wpostmeta.meta_key = 'pli'
 ";
 
 $pageposts = $wpdb->get_results($querystr, OBJECT);
 
if ($pageposts): foreach ($pageposts as $post): setup_postdata($post); 
  delete_post_meta($id, 'isproduct');
  add_post_meta($id, 'isproduct', '1');
  
  endforeach;
  endif;
}